1. Field of the Invention
The present invention relates to a system for automatically selecting a voice data transmission and reception system that allows the voice data transmission and reception system on an IP (Internet Protocol) network to be switched without necessity for the user to consider the state of the network.
2. Description of the Related Art
Conventionally, in a VoIP (Voice over Internet Protocol) system that transmits and receives voice on an IP network, communications according to the RTP (Real-time Transport Protocol) are known. The RTP is used as a transport layer protocol that accomplishes a real-time application especially through a network such as a video conference system and an Internet telephone. In the RTP, through an RTP port designated on a control channel (for example, sip port=5060), voice and a moving picture are transmitted and received in real time.
Generally, on an IP network in which a firewall is disposed between a LAN and the Internet, only IP port numbers used in predetermined protocols such as HTTP, FTP, and SMTP are permitted to be connected. The other port numbers are prohibited from being connected.
Each IP port number is represented by a two-octet positive number, which ranges from 0 to 65535. An application using an IP network communicates through one of these IP ports. Normally, each application is assigned one of the following IP port numbers.
WELL KNOWN PORT NUMBERS=0 to 1023
These port numbers are used for system (or route) processes of most systems and for programs of privileged users. Typical examples are ftp=21, http=80, etc.
REGISTERED PORT NUMBERS=1024 to 49151
These port numbers are used for general user processes of most systems and for programs executed by general users. Typical examples are call-sig-trans (H.323 AnnexE)=2517, sip=5060, etc.
DYNAMIC AND/OR PRIVATE PORTS=49152 to 65535
These port numbers can be freely used by users. Normally, VoIP control protocols such as H.323 and SIP provide RTP ports in this range and connect communication ports therethrough.
A firewall filters unnecessary IP ports of these ports so as to prevent external harmful attacks in advance. In particular, the DYNAMIC AND/OR PRIVATE PORTS are ports that tend to be filtered. On the other hand, generally, for a network access, at least http port (=80) is reserved.
To perform an IP voice communication through any port number in such a system, a system structure in which a multimedia proxy server is disposed has been disclosed (for example, refer to Patent Document 1).
In addition, an IP wireless phone technology for accessing an IP network through a wireless LAN or a PHS (Personal Handy-phone System) data communication network and for performing an IP voice communication has been disclosed (for example, refer to Patent Document 2).
[Patent Document 1]
Japanese Patent Publication No. 2002-9846-A
[Patent Document 2]
Japanese Patent Publication No. 2001-223748-A
However, in a conventional VoIP system, a communication according to RTP or another voice data transmitting and receiving method should be preset. In other words, it was impossible automatically to determine whether or not a voice data communication according to the RTP can be made and switch to a proper communication system.